As a result of the Clean Air Act Amendments of 1990, two “tiers” of emission standards for light-duty vehicles in the United States were defined. These standards specifically restrict emissions of carbon monoxide (CO), oxides of nitrogen (NOx), particulate matter (PM), formaldehyde (HCHO), and non-methane organic gases (NMOG) or non-methane hydrocarbons (NMHC). The Tier I standard was phased in from 1994 to 1997. Tier II standards are being phased in from 2004 to 2009. Within the Tier II standard, there are sub-rankings ranging from BIN 1-10.
To meet these standards, many advances have been made in engines and their control systems. New combustion control strategies are designed to minimize engine-out emissions and to control exhaust gas composition and temperature for optimum operation of post-combustion emissions control devices.
One such combustion control strategy is based on “airflow-based” control, especially designed for diesel engines or other engines that use direct fuel injection. “Airflow-based” control systems may be contrasted to more conventional “fuel-based” control systems. In fuel-based control, in response to activity of the accelerator pedal, the engine control unit determines the quantity of fuel to inject. Downward action of the accelerator pedal causes the engine control unit to inject more fuel. With this type of engine control, it is difficult to provide air-fuel ratios that are matched to desired combustion modes.
Airflow-based control systems are also referred to as “airflow dominant” control systems. In modern engines, the dynamics of fuel delivery are fast and can be controlled on a cylinder-by-cylinder basis. On the other hand, airflow is greatly affected by delays in the exhaust gas recirculation (EGR) path and by turbocharger lag. Airflow dynamics are slower and more difficult to control than fuel delivery. To achieve specific air-fuel ratio targets, in airflow dominant control systems, the fast fuel dynamics follow the slower airflow dynamics.
Airflow based control systems require accurate sensors and airflow models. The inputs to the control calculations include both engine operating inputs, such as accelerator pedal position and engine speed, as well as sensor inputs, such as airflow mass, intake temperature, and intake pressure. Accurate control outputs, such as commands to control fuel injection and air-handling devices, require accurate real time input measurements.